For example, Japanese Patent Application Laid-Open Publication No. 2013-192034 (Patent Document 1) discloses a technique of switching one of two RPL (Ring Protection Link) ports set to a closed state with a closed section interposed therebetween from the closed state to an open state for a predetermined period. Although failure detection of bit dependence is likely to be difficult in a CCM (Continuity Check Message) frame, this failure detection is made possible by causing a frame containing a checksum to flow for a predetermined period from one RPL port to the other RPL port.
Also, Japanese Patent Application Laid-Open Publication No. 2013-239909 (Patent Document 2) discloses a technique for enhancing fault tolerance in the case where multi-chassis link aggregation and ERP (Ethernet Ring Protection) are applied at the same time. Each of L2 switches constituting the ERP monitors a link fault between itself and a neighboring L2 switch by transmitting and receiving an ERP neighbor monitoring frame based on a CC (Continuity Check) function of Ethernet (registered trademark) OAM (Operation Administration and Maintenance) to and from the neighboring L2 switch.